Abstract: The transmission shaft of a certain type of coupling broke during use. The causes of the fracture of the transmission shaft were analyzed using methods such as macroscopic observation, chemical composition analysis, mechanical performance testing, scanning electron microscopy analysis, metallographic examination, and thermal acid corrosion test. The results show that there were flake defects inside the transmission shaft, and there were many sulfides and secondary cracks with intergranular characteristics in the material. Hydrogen atoms aggregated near sulfides, forming hydrogen traps. Under the combined action of tissue and internal stress, cracks initiated and expanded at the hydrogen trap site, forming hydrogen induced cracks, ultimately led to the fracture of the transmission shaft.